This invention relates to battery operated inverters and, in particular, to an inverter for driving an EL panel without producing a DC bias on the lamps in the panel.
As used herein, and as understood by those of skill in the art, “thick film” refers to one type of EL lamp and “thin film” refers to another type of EL lamp. The terms only broadly relate to actual thickness and actually identify distinct disciplines. In general, thin film EL lamps are made by vacuum deposition of the various layers, usually on a glass substrate or on a preceding layer. Thick film EL lamps are generally made by depositing layers of inks on a substrate, e.g. by roll coating, spraying, or various printing techniques. The techniques for depositing ink are not exclusive, although the several lamp layers are typically deposited in the same manner, e.g. by screen printing. A thin, thick film EL lamp is not a contradiction in terms and such a lamp is considerably thicker than a thin film EL lamp.
As used herein, an EL “panel” is a single sheet including one or more luminous areas, wherein each luminous area is an EL “lamp.” An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include phosphor particles or there can be a separate layer of phosphor particles adjacent the dielectric layer. The phosphor particles radiate light in the presence of a strong electric field, using relatively little current.
In the context of a thick film EL lamp, and as understood by those of skill in the art, “inorganic” refers to a crystalline, luminescent material that does not contain silicon or gallium as the host crystal. (A crystal may be doped accidentally, with impurities, or deliberately. “Host” refers to the crystal itself, not a dopant.) The term “inorganic” does not relate to the other materials from which an EL lamp is made.
EL phosphor particles are typically zinc sulfide-based materials, including one or more compounds such as copper sulfide (Cu2S), zinc selenide (ZnSe), and cadmium sulfide (CdS) in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. EL phosphors typically contain moderate amounts of other materials such as dopants, e.g., bromine, chlorine, manganese, silver, etc., as color centers, as activators, or to modify defects in the particle lattice to modify properties of the phosphor as desired. The color of the emitted light is determined by the doping levels. Although understood in principle, the luminance of an EL phosphor particle is not understood in detail. The luminance of the phosphor degrades with time and usage, more so if the phosphor is exposed to moisture or high frequency (greater than 1,000 hertz) alternating current.
A modern (post-1985) EL lamp typically includes transparent substrate of polyester or polycarbonate material having a thickness of about seven mils (0.178 mm.). A transparent, front electrode of indium tin oxide or indium oxide is vacuum deposited onto the substrate to a thickness of 1000Å or so. A phosphor layer is screen printed over the front electrode and a dielectric layer is screen printed over phosphor layer. A rear electrode is screen printed over the dielectric layer. It is also known in the art to deposit the layers by roll coating.
The inks used include a binder, a solvent, and a filler, wherein the filler determines the nature of the ink. A typical solvent is dimethylacetamide (DMAC). The binder is typically a fluoropolymer such as polyvinylidene fluoride/hexafluoropropylene (PVDF/HFP), polyester, vinyl, epoxy, or Kynar 9301, a proprietary terpolymer sold by Atofina. A phosphor layer is typically screen printed from a slurry containing a solvent, a binder, and zinc sulphide particles. A dielectric layer is typically screen printed from a slurry containing a solvent, a binder, and particles of titania (TiO2) or barium titanate (BaTiO3). A rear (opaque) electrode is typically screen printed from a slurry containing a solvent, a binder, and conductive particles such as silver or carbon. As long known in the art, having the solvent and binder for each layer be chemically the same or chemically similar provides chemical compatibility and good adhesion between adjacent layers; e.g., see U.S. Pat. No. 4,816,717 (Harper et al.).
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by an inverter that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
A suitable voltage can be obtained from an inverter using a transformer. For a small panel, a transformer is relatively expensive. The prior art discloses several types of inverters in which the energy stored in an inductor is supplied to an EL lamp as a small current at high voltage as the inductor is discharged either through the lamp or into a storage capacitor. The voltage on a storage capacitor is pumped up by a series of high frequency pulses from the inverter. Capacitive pump circuits are also known but not widely used commercially.
The direct current produced by inverter must be converted into an alternating current in order to power an EL lamp. U.S. Pat. No. 4,527,096 (Kindlmann) discloses a switching bridge for this purpose. The bridge acts as a double pole double throw switch to alternate current through the EL lamp at low frequency. U.S. Pat. No. 5,313,141 (Kimball) discloses an inverter that produces AC voltage directly. A plurality of inverters are commercially available using either technology.
In general, inverters produce voltages that are only approximations of sinusoidal alternating current. In particular, the positive and negative half cycles of current are not necessarily identical. The result is a DC bias on an EL lamp that causes ionic migration from the phosphor layer and silver migration from the rear electrode, if silver particles were used. It is known in the art to use a DC blocking capacitor in series with an EL lamp; e.g. see U.S. Pat. No. 5,347,198 (Kimball). It is known in the art to use barrier layers to prevent or to impede silver migration; e.g. see U.S. Pat. No. 6,445,128 (Bush et al.).
As noted in the Kimball patent, a capacitor has a much higher leakage resistance than an EL lamp. Thus, the DC voltage drop across an EL lamp connected in series with a capacitor is minimal. As also noted in the Kimball patent, there is a miniscule current flowing through an EL lamp even in the “off” state, i.e. when a driver is turned off without fully discharging the lamp. The miniscule current, corresponding to a very small DC bias, has been found to cause ionic migration.
It is also known in the art to control the discharge of an EL lamp to simulate alternating current (e.g. U.S. Pat. No. 5,886,475; Horiuchi et al.), to reduce acoustic noise emitted by an EL lamp (e.g. U.S. Pat. No. 6,555,967; Lynch et al.), or to recover energy from an EL lamp (e.g. U.S. Pat. No. 5,982,105; Masters). While controlled discharge is known, such is not the same as discharging to zero volts. For example, circuits that are concerned with acoustic noise from an EL lamp only reduce the voltage across the lamp to a certain level, below which an abrupt change in voltage causes inaudible noise, if any noise at all. The abrupt change is not necessarily to zero volts and can leave a residue of as much as ten or twelve volts.
Other types of circuits have the same problem. Any circuit that uses pumping (whether capacitive or inductive) faces diminishing returns, i.e., less charge per pump cycle as an EL lamp discharges. The result is that pumping stops before zero volts is reached and the lamp is not fully discharged. Circuits that appear to be balanced or symmetrical, such as an “H-bridge” output, are not. Processing variations cause transistors to switch at slightly different voltages. Carefully matched or compensated switching elements are too expensive in the market for DC inverters for EL lamps. The result is DC bias on an EL lamp. Even a small DC bias is harmful, causing shortened life compared to properly driven lamps.
In view of the foregoing, it is therefore an object of the invention to provide a power supply for driving an EL panel from a battery without producing a DC bias on the panel.